All insurance is a form of "Ponzi scheme" inasmuch as it depends on an ever-growing influx of premiums in order to survive. Social Security, however, has a pre-determined date to begin its payout and an uncertain end date. The pay-in time is, on average, greater than the pay out time. The money is invested in VERY conservative ways, (or at least should be) and, thus, the cost of maintaining the system can be an issue. The whole argument that there are more people receiving SS is crap, the more people paying in the better. Also, it doesn't need to compete with other plans so faulty underwriting is not an issue. So, barring a drastic increase in the cost of managing the system, there should not be a problem with its survival.
The fact that people are living longer only means that there needs to be an adjustment to the actuarial model such that the return on assets must increase, and if you have more people contributing (hence, a larger pot), that is easily attained.

"The liberals can understand everything but people who don't understand them."
Lenny Bruce

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Dostoevsky